linux-kernel - Re: [PATCH 12/25] Remove a branch by assuming __GFP_HIGH == ALLOC

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Date:	Tue, 21 Apr 2009 10:46:22 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 12/25] Remove a branch by assuming __GFP_HIGH ==
 ALLOC_HIGH

On Mon, 2009-04-20 at 23:19 +0100, Mel Gorman wrote:
> Allocations that specify __GFP_HIGH get the ALLOC_HIGH flag. If these
> flags are equal to each other, we can eliminate a branch.
> 
> [akpm@...ux-foundation.org: Suggested the hack]

Yikes!

> Signed-off-by: Mel Gorman <mel@....ul.ie>
> ---
>  mm/page_alloc.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 51e1ded..b13fc29 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1639,8 +1639,8 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
>  	 * set both ALLOC_HARDER (!wait) and ALLOC_HIGH (__GFP_HIGH).
>  	 */
> -	if (gfp_mask & __GFP_HIGH)
> -		alloc_flags |= ALLOC_HIGH;
> +	VM_BUG_ON(__GFP_HIGH != ALLOC_HIGH);
> +	alloc_flags |= (gfp_mask & __GFP_HIGH);

Shouldn't you then also change ALLOC_HIGH to use __GFP_HIGH or at least
add a comment somewhere?

>  
>  	if (!wait) {
>  		alloc_flags |= ALLOC_HARDER;

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